Thermal Conductivity of the Hole-Doped Spin Ladder System ${\mathrm{Sr}}_{14-\mathit{x}}{\mathrm{Ca}}_{\mathit{x}}{\mathrm{Cu}}_{24}{O}_{41}$

The thermal conductivity of the spin-1/2 ladder system ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ ( $x=0\mathrm{}$, 2, and 12) has been measured both along ( ${\kappa }_c$) and perpendicular to ( ${\kappa }_a$) the ladder direction at temperatures between 5 and 300 K. While the temperature dependence of ${\kappa }_a$ is typical for phonon heat transport, an unusual double-peak structure is observed for ${\kappa }_c\left(T\right)$. We interpret this unexpected feature as a manifestation of quasi-one-dimensional magnon thermal transport mediated by spin excitations along the ladders.